After Studying this section, you should be able to: • Recognise BJT astable multivibrator circuits. • Understand the operation of astable multivibrators. • Calculate. An Astable Multivibrator or a Free Running Multivibrator is the multivibrator which has no stable states,oscillates continuously between two unstable states. A multivibrator is a device that switches between two states. It is a type of oscillator and can be used as a trigger, converter, moderator, or divider. It.

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Q1 is on and connects the left-hand positive plate of C1 to ground. It is implemented by the coupling capacitors that instantly transfer voltage changes because the voltage across a capacitor cannot suddenly change.

Module 4.1

This is due to resistance within the circuit, lack of a completely closed astablle at the output terminals, and the tendency for one capacitor or transistor to absorb astablw at a slightly different rate than the other. The pulse width T of a monostable multivibrator is calculated as follows: With TR1 conducting, its base would have been about 0.

If further trigger pulses do not affect the period, the circuit is a non-retriggerable multivibrator.

If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. The frequency of oscillation can be calculated, as the time for the relevant capacitor to charge sufficiently for a change of state to take place, will be approximately 0. The circuit has two astable unstable states that change alternatively with maximum transition rate because of the “accelerating” positive feedback.

Because of very strong feedback signal, the transistors are driven either to saturation or to cutoff. It is a type of oscillator and can be used as a trigger, converter, moderator, or divider.


A free-running multivibrator with a frequency of one-half to one-tenth adtable the reference frequency would accurately lock to the reference frequency. However, if the circuit is temporarily held with both bases high, for longer than it takes for both capacitors to charge fully, then the circuit will remain in this stable state, with both bases at 0.

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Views Read Edit View history. The rising collector current in Q1 drives its collector more and more positive. Multkvibrators de Physique in French.

Time bases scanning generators: It is also called free-running relaxation oscillator. To approach the needed square waveform, the collector resistors have to be low in resistance. The period of time during adtable Q2 remains at saturation and Q1 at cutoff is determined by C2 and R2. The output of transistor Q1 is coupled to the input of Q2 through C1 while the Q2 is fed to the input of Q1 through C2.

Since it produced a square wavein contrast to the sine wave generated by most other oscillator circuits of the time, its output contained many harmonics above the fundamental frequency, which could be used for calibrating high frequency radio circuits. This latch circuit is similar to an astable multivibrator, except that there is no charge or discharge time, due to the absence of capacitors.

As C1 discharges, the base bias at Q2 becomes less positive and at a time determined by R1 and C1, forward biased is re-established at Q2. This page was last edited on 20 Novemberat When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state.

BJT Astable Multivibrators

Hence the base of Q1 will become more positive than Q2. Chaos 22 Retrieved from ” https: Now, the capacitor C2 is fully charged in the previous State 1 to the power supply voltage V with the polarity shown in Figure 1. A change of state has occurred at both outputs. The circuit is useful for generating single output pulse of adjustable time duration in response to a triggering signal. multivvibrators


The charges developed across C1 and C2 are sufficient to maintain the saturation and cutoff conditions at Q1 and Q2 respectively. The duration of state 1 low output will be related to the time constant R 2 C 1 as it depends on the charging of C1, and the duration of state 2 high output will be related to the time constant R 3 C 2 as it depends on the charging of C2. Got Something To Say: The circuit keeps on changing state in this manner producing a square wave at each collector.

One has high voltage while the other has low voltage, except during the brief transitions from one state to the other. Suppose that at switch on, TR1 is conducting heavily and TR2 is turned off. It is considered below for the transistor Q1.

As its right-hand negative plate is connected to Q2 base, a maximum negative voltage – V is applied to Q2 base that keeps Q2 firmly off.

After elapsing the time, it returns to its stable initial state. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal. Pulse And Digital Circuits.

Because they do not need to be the same, an asymmetric duty cycle is easily achieved. Bistable multivibrator Astable multivibrator It is also called free-running relaxation oscillator.

Multivibrator – Wikipedia

This further increases the collector current in Q2 and cause decrease of collector current Q2. Related Posts Butterworth Filter. This feedback is supplied in such a manner that one transistor astahle driven to saturation ON state and other to cut-off OFF state.